1. Field of the Invention
The present invention relates to security cameras, and, more particularly, to security cameras that undergo pan and tilt movements.
2. Description of the Related Art
Surveillance camera systems are commonly used by retail stores, banks, casinos and other organizations to monitor activities within a given area. The cameras are often provided with the capability to pan and tilt in order to acquire images over a wide domain. The tilt of the camera generally refers to the pivoting of the camera about a horizontal axis that is parallel to the floor, such that the lens of the camera may tilt between an upwardly pointing position and a downwardly pointing position. The pan of the camera refers to the rotation of the camera about a vertical axis that is perpendicular to the floor, such that the lens may scan from side to side. The cameras may also be able to zoom in order to reduce or enlarge the field of view. Oftentimes, each camera is linked to video display units in a security surveillance room with surveillance personnel monitoring the multiple video display units.
In a dome camera system there are typically two axes of motion, including a pan axis and a tilt axis. Dome cameras that use stepper motors are usually an open loop system dependent on the torque of the motor to retain the correct position. However, at power up the camera's central processing unit (CPU) does not know the camera's PT (pan and tilt) position, so the CPU must have some method of finding a start point or a home position. Typically, a home position is a single point in the dome's hemisphere, and this point is found using two sensors, one sensor being for pan and the other sensor being for tilt.
An open-loop motor control system utilizes electronic pulses to move a stepper motor(s) a certain number of rotational steps, and does not have any feedback device to verify the commanded position was reached. In spite of no feedback, this open-loop system works well due to the robust positioning properties of properly designed stepper motors and their associated control electronics.
It is necessary for a pan-tilt-zoom surveillance camera to be able to accurately acquire pre-set viewing angles after loss of and subsequent restoration of power to the unit. Since most motors do not retain their position when de-energized, or include means to accurately freeze the system position upon power removal, a means to provide a consistent restart is needed. It may be easier to provide a means for the system to go through a “homing” or “calibration” routine to establish a consistent starting point from which control electronics can then provide reproducible positions.
U.S. Pat. No. 5,627,616 assigned to Philips Electronics (predecessor to Bosch Security Systems) discloses an open-loop control of pan and tilt motors on a pan-tilt-zoom (PTZ) camera. The patent teaches calibration of the pan home position through use of an optical homing sensor, called a pan motor switch, and actuated by a flag. This flag/sensor approach places design constraints on the camera system. First, a pair of flag/sensors must be provided for both pan and tilt axes. Second, wiring must be provided to the sensor location. Third, clearance must be provided to allow the flag to rotate 360 degrees.
In contrast, some PTZ cameras utilize encoders to determine rotational position for closed-loop control. U.S. Pat. No. 6,715,940, assigned to General Electric, teaches the use of an optical sensor to read a pseudorandom or chain code encoding pattern printed on a bearing race. Such a scheme requires a high degree of precision, which is expensive to achieve.
What is neither disclosed nor suggested by the prior art is a method of establishing a home pan position and a home tilt position of a surveillance camera that overcomes the above-described problems with the prior art.